The invention relates, in general, to an apparatus for converting chemical energy into electrical energy through the process of electrolysis of water. Specifically, the invention describes an apparatus for the implementation of a passive, variable pressure regenerative, fuel cell system. During power drawing operations, a fuel cell generates energy by combining hydrogen and oxygen gas to form water. During charging operations, an electrolyzer is used to convert water back into its constituent parts; hydrogen and oxygen.
Configuration of a Typical Fuel Cell
A typical electricity generating fuel cell is shown schematically in FIG. 1. Each cell is comprised of an anode chamber 100 having a porous anode 105 and a cathode chamber 110 having a hydrophobic porous cathode 115 separated by an electrolytic membrane 120, known as a proton exchange membrane (PEM). As is well-known to those of ordinary skill, this membrane may be an acid or a solid polymer such as Nafion.RTM. (a trademarked product of E. I. DuPont de Nemours of Wilmington, Del., which is a polymer of polytetrafluoroethylene with fluorinated ether side chains terminated with sulfonic acid groups). The PEM material is designed to readily permit the transport of ions and solvent between the anode and cathode chambers, but to be relatively impermeable to gas.
During power generation at terminals 135, hydrogen is typically applied to a fuel cell's anode 105 via an opening 125 and oxygen 140 to the cell's cathode 115, with water 130 forming in the cell's cathode chamber 110 from oxidation of the hydrogen. On the other hand, during charging operations, water is typically applied to the cell's anode 105, with oxygen being extracted from the cell's anode chamber 100 via an opening 125, and hydrogen 130 being extracted from its cathode chamber 110.
Basic Operational Components of a Fuel Cell System
Referring to FIG. 2, it is well-known to those of ordinary skill that fuel cell systems have long been used in specialized electrical power generation applications such as spacecraft. In such systems, a plurality of fuel cells (e.g., a fuel cell system) can behave as a monolithic battery. A battery 200 includes a fuel cell 205 and additional supporting equipment (not shown). The battery 200 delivers electrical power over a power line 210 to electrical loads such as lights 215, radios 220, and so forth.
As shown in FIG. 3, a basic passive, variable pressure, regenerative fuel cell system 200 from the prior art comprises a regenerative fuel cell component 205 which sits above a main water tank 300, an electrolyzer 305 which sits below the main water tank 300, a small secondary water tank 310, a water flow restriction orifice or fluid flow valve 315 communicating with the main water storage tank 300 and the secondary water storage tank 310, a liquid gas separator 320, a gaseous hydrogen storage tank 325, and a gaseous oxygen storage tank 330. Such fuel cell systems commonly use a multiplicity of fuel cells which are designed, for example, to deliver 30 kilowatt-hours of electrical energy at a nominal power level of 2.50 kilowatts at 120 volts. (For convenience, the expression "fuel cell" is used to represent an element comprised of one, or more, individual fuel cells unless otherwise noted.)
Typically, a hydrogen line 335 from the hydrogen supply tank 325 is connected to both the fuel cell 205 and electrolyzer 305. In this configuration, hydrogen is supplied to the anode side of the fuel cell 205 (during power generation operations) and is extracted from the cathode side of the electrolyzer 305 (during recharge operations). An oxygen line 340 is attached to the water tank 300 which, via line 345, directs oxygen to the fuel cell's 205 cathode.
The fuel cell, water tank, and electrolyzer are stacked vertically as shown so that liquid water produced at the fuel cell's cathode (where electrons are consumed) is concurrently drained directly into the main water tank 300 through line or conduit 345 via gravitational forces, while water stored in the main water tank can be, in turn, gravitationally fed to the anode side of the electrolyzer (where electrons are produced) through line or conduit 350.
During recharge operations, oxygen produced at the electrolyzer's 305 anode passes through line 350 and into water tank 300 where it bubbles up, through line 340, to the oxygen storage tank 330. Concurrently, hydrogen produced at the electrolyzer's cathode is returned to the hydrogen storage tank 325 through a liquid-gas separator 320.
Individual cells of both the fuel cell and electrolyzer are sometimes interconnected in a series arrangement, often called a "stack." The number of cells in this series is determined by the desired DC voltage. Under fully charged conditions (approximately 21 MPa or 3,000 psi) the open circuit DC voltage of each individual fuel cell is approximately 1.3 volts while under fully discharged conditions (approximately 0.7 MPa or 100 psia) the open circuit DC voltage of each cell is approximately 1.2 volts.
Problem of Fuel Cell PEM Membrane Drying
A major concern in a fuel cell system such as that shown in FIG. 3 is to prevent dryout, and the devolatilization and cracking of the fuel cell's 205 PEM anode surface that may result, during brief periods of water feed interruption to the anode chamber 100 which are made possible by the passive operating nature of this electrochemical device. Dryout can occur because aqueous protons migrating across the PEM 120 during normal fuel cell operation (from anode side to cathode side) carry liquid water molecules along with them which, if not replenished, will lower the water concentration on the PEM's anode surface. Dryout prevention is often accomplished by holding a reservoir of excess water in close contact with the fuel cell's PEM anode surface. Any PEM dryout at high cell pressures will quickly lead to electrolyte oxidation and subsequent fuel cell failure.
Some Prior Approaches to the PEM Dryout Problem
One prior approach to solving the problem of proton exchange membrane dryout in regenerative fuel cells is proposed in U.S. Pat. No. 4,657,829 to McElroy et al. McElroy et al. propose a hydrogen/air fuel cell having a water electrolysis subsystem and gas storage system. The electrolysis subsystem comprises a plurality of bipolar cells. Water is introduced into the anode chambers of the electrolysis cells from liquid/vapor separators. Hydrogen and oxygen produced by the cells are fed to these liquid/vapor separators. Excess water from the anode chamber and water pumped via electoosmosis across the membrane with the hydrogen ions, is separated from the gases and introduced into respective pressurized storage vessels. Each storage vessel contains a float switch which actuates pumps and drain valves to feed water to the electrolyzer. As the oxygen and hydrogen in the pressure vessels are consumed, their pressure drops and a signal from a pressure transducer causes additional water to be electrolyzed to replenish depleted oxygen and hydrogen levels. The electrolyzer system associated with a fuel cell stack described above is potentially limited, however, by a multiplicity of possible switching and fluid flow control functions.
Another proposed solution is set out in U.S. Pat. No. 5,064,732, to Meyer. The Meyer '732 patent proposes disposing a porous element between two adjacent cells of a fuel cell stack, namely the cathode chamber of a first cell and the anode chamber of a second cell. A pressure differential is maintained across the porous element to cause water to pass through the porous element from the cathode chamber to the anode chamber. A potential detriment with the Meyer '732 patent is that the cathode chamber must always be held at a higher pressure than the anode chamber. This can prevent the operation of the electrolyzer during power generation operations.
The prior approaches discussed above share some common general problems. Most notably, a variety of additional equipment is needed to address the problem of PEM dryout. Additional pumps, valves, compressors, and so forth add to the cost of systems, increase their weight, and contribute to whatever unreliability problems may already exist. (It is a well-known fact of engineering that the greater the number of components involved in a system, in general the shorter the mean time to failure of the system.)
FIG. 4 shows another prior an approach that makes use of pressure differentials between the hydrogen storage tank 325 and the oxygen storage tank 330. During fuel cell operation, the temperatures in the tanks are controlled via temperature control elements 400 so that the hydrogen storage tank is at a lower pressure than the oxygen storage tank so that water will flow from the water tank into the anode side of the fuel cell. During electrolyzer operation on the other hand, the pressures in the tanks are reversed by the temperature control elements 400 so that water, produced at the cathode side of the electrolyzer, is allowed to flow back into the water tank, facilitated by having the hydrogen storage tank 325 pressure higher than the oxygen storage tank 330 pressure. As with the other prior approach techniques discussed, this solution requires extra equipment to implement and, therefore, suffers from the same cost and reliability drawbacks as the previously cited prior art designs.